1. Field of the Invention
This invention relates generally to composites made by layer-by-layer assembly materials from dispersions of highly anisotropic colloids.
2. Background
Nanoscale dispersions have been investigated as new materials for traditional electronic and optical applications, chemical and biological sensors, and as building blocks for conceptually novel molecule-based devices. In many applications, nanoscale dispersions are used as composite materials whose properties, among other factors, can be controlled by the aspect ratio of the particles in dispersion. As used herein with respect to a particle, the term “high aspect ratio” refers to a non-spherical species having at least one dimension differing from the others by a ratio of at least 1:1.5. An increase of the particle anisotropy can lead to new quantum mechanical effects in the nanoscale dispersions as well as to improved mechanical, optical and electrical properties of the composite materials made from the nanoscale dispersions. The preparation of such composites and their applications differ significantly from nanoscale dispersions and also differ significantly from layer-by-layer assembly of spherical or low-anisotropy colloids.
Examples of such composites from highly anisotropic materials include composites prepared from metal and semiconductor nanorods and carbon nanotubes. These composites are expected to have much higher diffusion rates of charge carriers through the composites and significantly improved mechanical strength. However, preparation of such composites, especially from carbon nanotubes, is a longstanding and difficult problem due to poor miscibility of high aspect ratio particles with other compounds such as polymers. Although miscibility can be partially improved by surface modification of the nanotubes and other means, a tendency for separation still occurs at high nanotube loadings. Alignment of carbon nanotubes in the polymer matrix leads to substantial improvement of electrical, mechanical, and optical properties.
Thin film technology, wherein inorganic particles with sizes on the order of 1-100 nm are arranged in layers to form a film, is being used presently for an increasingly large number of different technological applications, including, among other things, information storage systems, chemical and biological sensors, fiber-optical systems, magneto-optical and optical devices, pervaporation membranes, protective coatings and light emitting diodes. Current techniques for preparing such films include chemical vapor deposition (in which no discrete inorganic particles are involved), sol-gel technology (producing porous materials that can be sintered to get uniform films), or deposition from colloidal dispersions (spin-coating, dip-coating, Langmuir-Blodgett deposition, etc.). Layer-by-layer assembly (LBL) is a method of thin film deposition, which is often used for oppositely charged polymers or polymers otherwise having affinity and has recently been applied to the preparation of thin films of nanoparticles. Its simplicity and universality, complemented by the high quality films produced thereby, make the layer-by-layer process an attractive alternative to other thin film deposition techniques. LBL can be applied to a large variety of water-soluble compounds and is especially suitable for the production of stratified thin films in which layers of nanometer thickness are organized in a specific predetermined order.
It is thus an object of the present invention to provide a method for the assembly of materials that incorporate highly anisotropic colloids using a layer-by-layer process.
It is a further object of the invention to circumvent the tendency for separation of nanotubes at high nanotube loadings.
A further object of the present invention is to provide a method for the assembly of materials that incorporate aligned highly anisotropic colloids using a layer-by-layer process.
A further object of the invention is to provide a method or carbon nanotube alignment, which among other advantages makes possible the preparation of unique criss-crossed composites. The cross-linked bonds may be formed during the deposition of layers, between the deposition of layers and after all desired layers have been deposited or after the film has been removed from the substrate.
The preparation and utilization of ultra strong composites from highly anisotropic colloids is also an object of this invention.